has many :through nested attributes with checkboxes

edit.html.haml

# Using formtastic but can easily converted to use standard rails form helpers

= semantic_form_for [ @user ] do |f|

  = f.semantic_fields_for :user_specialities do |user_speciality|
    = user_speciality.inputs do
      = user_speciality.input :speciality_id, :as => :hidden
      = user_speciality.input :_destroy, :label => user_speciality.object.speciality.name, :as => :boolean, :checked_value => 0, :unchecked_value => 1, :input_html => { checked: !user_speciality.object.new_record? }

models.rb

# user.rb
class User < ActiveRecord::Base

  has_many :user_specialities, :dependent => :destroy
  has_many :specialities, :through => :user_specialities

  attr_accessible :user_specialities_attributes
  accepts_nested_attributes_for :user_specialities, :allow_destroy => true
end

# specialities.rb
class Specialities < ActiveRecord::Base

  has_many :user_specialities, :dependent => :destroy
  has_many :specialities, :through => :user_specialities
end

# user_specialities.rb
class UserSpecialities < ActiveRecord::Base

  belongs_to :user
  belongs_to :speciality
end

user_controller.rb

class UserController < ApplicationController

  def edit
    @user = current_user
    build_unpicked_specialities
  end

  def update
    @user = current_user
    @user.update_attributes(params[:user])
    respond_with @user
  end
 
  private

  # Builds and sorts unpicked specialities for a user to be used in the nested attribute
  # Returns an array of user specialities sorted by speciality position
  def build_unpicked_specialities
    (Speciality.all - @user.specialities).each do |speciality|
      @user.user_specialities.build(:speciality => speciality)
    end

    @user.user_specialities.sort_by! {|us| us.speciality.position }
  end

end